1. Field of the Invention
The present invention relates to a mechanism for preventing reversal of rotation of a moving member which moves in both the normal and reverse directions, and more specifically, to a mechanism for preventing reversal of rotation of a film cartridge spool of a camera based on the advanced photo system (APS).
2. Description of the Prior Art
Generally, a ratchet mechanism is used for prevention of reversal of rotation and positioning of a moving member which moves in one direction. However, in the ratchet mechanism, since engagement by ratchet teeth occurs every interval between ratchet teeth, it is impossible to stop a moving member at a given position and it is also impossible to easily stop at a predetermined position a moving member which is moving or rotating in both the normal and reverse directions.
For example, in an APS camera, the film cartridge is provided with a function to indicate whether the film loaded in the film cartridge is unused, exposed partway, exposed to the end or developed. The so-called use condition mark is called a visual exposure indicator (VEI) and the use condition is indicated by stopping the cartridge spool at a specific rotation stop position. Therefore, in an APS camera, it is necessary for the rotation stop position of the cartridge spool to be different, for example, between when the film is rewound after exposed to the end and when the film is rewound after exposed partway.
The film cartridge of an APS camera is provided with a cartridge door, namely a light lock door (LLD). By closing the LLD, shielding of the film exit is performed and the cartridge spool is engaged, and by opening the LLD, the film exit is opened and the cartridge spool is disengaged. However, since the cartridge spool is not engaged from when the cartridge spool is stopped at a predetermined rotation stop position after the film is rewound to when the cartridge door is closed, there is a possibility that the cartridge spool is rotated. When this happens, an erroneous indication is provided such that a film which is exposed only partway is regarded as exposed to the end.
Moreover, since back tension is applied to the film when the film is rewound, the film is tightly wound in the cartridge spool. In an APS camera, since it is necessary to load the film after pushing the film out of the film cartridge, the film is made firm, so that the film in the film cartridge after being rewound is like a so-called spiral spring. If the cartridge spool is disengaged under this state, it is rotated in a direction opposite to the rewinding direction because of the resilience of the film.
Improvements have been made to solve this problem. An example of the improvements is such that a frictional force in the direction of axial rotation is always imposed by an O ring and the like made of a resilient material such as rubber on a camera-body-side fork engaged with the cartridge spool for performing loading, winding and rewinding of the film wound around the cartridge spool. With this arrangement, the fork does not idle even after the rotation drive force transmitted to the fork is stopped, so that erroneous rotation of the cartridge spool is prevented.
Another example is such that a fork gear for transmitting a rotation drive force to the fork is engaged with a lever. Specifically, as shown in FIG. 18, the lower end of a longitudinally elongated lever 31 is engaged with a fork gear 30 which moves in conjunction with a non-illustrated fork, and an elongate hole 31a is provided in the lever 31 along the length so as to be rotatably and slidably engaged with a pin 32 provided on the camera body side.
A coil portion of a torsion coil spring 36 is engaged with a pin 34 provided on the body side, one arm portion of the spring 36 is fixed to a pin 35 and the other arm portion pushes by a spring force a pin 33 provided at the upper end of the lever 31 in the direction of the arrow, so that the lever 31 is pressed against the fork gear 30 to impose a load thereon.
At this time, when the fork gear is continuously rotating leftward in the figure, the lever 31 is swung leftward as shown by the broken line of the figure, so that one side surface thereof slides over the tips of the teeth of the fork gear 30. When the rotation direction of the fork gear 30 is reversed from the leftward direction to the rightward direction, the lever 31 is once engaged with the fork gear 30 as shown by the solid line of the figure, pushed out rightward by further rotation, and swung rightward as shown by the dash and dotted line of the figure, so that the other side surface of the lever 31 slides over the tips of the teeth of the fork gear 30.
However, in the above-described structure in which a frictional force is always applied in the direction of axial rotation of the fork, it sometimes occurs during the rewinding of the film by the spool on the camera side that the film is momentarily wound very tightly around the cartridge spool to change the rewinding speed for an instant. When the film is rewound, information on photographing and the like is magnetically recorded on the film. However, if the rewinding speed changes, the magnetic signals become nonuniform, so that intended information cannot be recorded.
In the above-described structure in which the lever is engaged with the fork gear, a load is generated when the lever is pushed out by the fork gear; however, since the fork gear and the lever are not always engaged with each other at the moment of reversal of the rotation, no load is generated and the fork idles at that time, so that there is a possibility that the cartridge spool rotates erroneously. Moreover, a sound of collision between the lever and the fork gear is generated when the fork gear is driven. This is unpleasant and causes wear of the lever and the fork gear.